1. Field of the Invention
The invention relates to the casting of metal alloys which have been formed into a thixotropic suspension as part of their preparation.
2. Description of the Prior Art
A suspension of this kind is obtained by heating a metal alloy to a temperature significantly higher than the liquidus temperature so that it becomes totally liquefied and then agitating it while its temperature reduces to a value between the liquidus temperature and the solidus temperature. The dendrites which tend to form during such cooling are transformed by such agitation into approximately spherical globules.
The state of the art is indicated by U.S. Pat. Nos. 3,902,544 and 3,948,650.
In the first of these patents (U.S. Pat. No. 3,902,544), it is stated that the thixotropic metal alloy may be continuously cast in ingot form, or injection molded in a metal mold, pressurized by a piston driven by a ram, or shaped to the required final form by compression between the two parts of a diestamping die.
In the second of the aforementioned patents, the casting of the thixotropic alloy into ingot molds to obtain ingots is envisaged. Such ingots may then be re-heated to a temperature between the solidus and liquidus temperatures and then shaped to the required form by a process such as stamping or forging.
It is advantageous to shape metal alloys to their final form from a thixotropic suspension since the difference between the intermediate temperature at which they are formed and their final temperature in the solid state is reduced, so that the risk of shrinkage and cracking during cooling is considerably reduced.
The merit of the invention consists in the recognition that it is possible to cast an alloy in the thixotropic state directly into a mold under the effect of centrifugal force. This avoids the necessity for the intermediate stage of casting an ingot, which has the following disadvantages:
the further energy consumed to re-heat the ingots, PA1 the slowness of the process, which is effected in two stages, PA1 the further and relatively high consumption of energy for final shaping.
Also avoided is the use of shaping methods necessitating bulky and expensive tooling, such as stamping and injection molding under pressure.